circuits chapter 35. lab 21 what will happen to bulbs 1 and 2 when you disconnect the wires at...

CircuitsChapter 35

LAB 21What will happen to

bulbs 1 and 2 when you disconnect the wires at various points?

Consensus: Current requires a closed loop

LAB 22What type of object, when

inserted into the loop, will allow the two test bulbs to light?

Consensus: Current requires a closed loop made entirely of conductors.

LAB 23What parts of a socket and

bulb are conductors and which are insulators? What is the conducting path through the bulb?

Fig 2. Clip – side view

Base

Clips

Plates

Consensus: Trace the Continuous Conducting Path (CCP) through the bulb.

SchematicsSchematics are drawings used to show circuits

using symbols instead of actual drawings.

Here are some examples:

SchematicsYou have a list of several

useful symbols in your book. Here are a few more:

SchematicsDraw a schematic of a circuit consisting of a battery made

of 2 cells (label + and – ends) and two light bulbs.

SchematicsDraw a schematic of a circuit consisting of a battery made

of 3 cells (label + and – ends), a light bulb and a resistor.

SchematicsDraw a schematic of a circuit consisting of a battery

(label + and – ends), three light bulbs, an ammeter and a voltmeter on the second bulb.

SchematicsA visual representation of a circuit is shown below. On the

right, sketch the corresponding schematic diagram.

R

R

R

R

Worksheet 2

Worksheet 2

3. bulb 1: Lit Unlit

bulb 2: Lit Unlit

Worksheet 2


bulb 2: Lit Unlit

CIRCUIT LAB 2How does the number of bulbs in

a single loop affect the overall resistance of the circuit?

Based on the measurements of the ammeter and observations of bulb brightness, what can you state about the flow rate in each circuit?

R

R

R

A

A

A

CIRCUIT LAB 2Consensus:

As resistance in a single closed loop goes up, the current goes down.

V = IR

New Terms:Series circuit – a circuit with all sources of resistance on a single loop with the battery.

R

R

R

A

A

A

Series CircuitsSeries circuits – circuits where all sources of

resistance are on a single closed loop with the battery.

Series CircuitsTotal Resistance = the sum of all resistances.

R = R1 + R2 + R3…

This means that two 8- resistors could be replaced with one 16- resistor.

Activity 25Va

V b

d

L

L V c

V

Va

V b

d

c

V

R

R

R

R

V

V

V e

f

a

V b

d

V c

V

R

L

V

a

V b

d

V c

V

R

L

V

Consensus:

For a series circuit, the voltage across the whole circuit is equal to the sum of the individual voltage drops.

Activity 25a

b

c

A

A

A

a

b

c

L

L

b

c

A

A

R

a

b

c

d

e

A R

R

A

A Rde

a

a

b

c

A

A

AR

a

b

c

La

b

c

A

A

A

R a

b

cL

Consensus:

For a series circuit, the total current flows through each resistance on the loop.

Series CircuitsTotal Resistance = the sum of all resistances.

R = R1 + R2 + R3…

This means that two 8- resistors could be replaced with one 16- resistor.

Series CircuitsVoltage drop = the voltage pulled from the

battery by a resistance to produce current.

Total voltage = the sum of the voltage drops across each resistance.

“Rules” for Series CircuitsStep 1: Total resistance = sum of all resistances

R = R1 + R2 + R3 + …

Step 2: Total current is the same throughout

I = V/R

Step 3: Voltage drop – use Ohm’s Law

V1 = I (R1)

Check: Total voltage = sum of voltage drops = battery

V = V1 + V2 + V3 + …

Series Circuits

Suppose you have three 3- resistors connected in series to a 9-V battery.

What is the total resistance?

9

What is the current in the circuit?

1 A

What is the voltage drop across each resistor?

3 V

Series CircuitsThere are three 20- resistors connected in series across

a 120-V generator. What is the equivalent resistance of the circuit? What is the current of the circuit? What is the voltage drop across each resistor?

R = R1 + R2 + R3…R = 20 + 20 + 20R = 60

I = V/RI = 120V/60I = 2 A

V = IRV = 2A(20)V = 40 V

Series CircuitsA 10- resistor, a 15- resistor and a 5- resistor are connected

in series across a 90-V battery. What is the equivalent resistance of the circuit? What is the current in the circuit? What is the voltage drop across each resistor?

R = R1 + R2 + R3…R = 10 + 15 + 5R = 30

I = V/RI = 90V/30I = 3 A

V = IRV = 3A(10)V = 30 V

V = IRV = 3A(15)V = 45 V

V = IRV = 3A(5)V = 15 V

V = 30V + 45V + 15V = 90V

CIRCUIT LAB 3

How does the number of bulbs along side affect the overall resistance?

Based on the ammeter reading and bulb

brightness, what can we state about the flow rate through each bulb?

CIRCUIT LAB 3

Consensus:

Bulbs alongside create their own closed loop increasing the amount of current overall and decreasing total resistance. This is the opposite of bulbs in series.

New Terms:

Parallel circuit – a circuit with each source of resistance on its own closed loop with the battery.

Parallel CircuitsParallel circuits – circuits where each source of

resistance is on its own closed loop with the battery.

Parallel CircuitsTotal Resistance = sum of the inverses of each

resistance

1/R = 1/R1 + 1/R2 + 1/R3…

This means that two 8- resistors in parallel could be replaced by one 4- resistor.

Activity 27

Va V Vb cL L a V bL V cL V dL V eLV

Consensus:

The voltage drop across each lamp is equal to the total across the battery.

Activity 27a

b c

A

Aa

b

c

d

L

A

L

A

d

b c

A

A

A

A

f

L L

dA

L

eA

L

a

Consensus:

The total current in the circuit is equal to the sum of the currents in each lamp.

Parallel CircuitsTotal Resistance = sum of the inverses of each

resistance

1/R = 1/R1 + 1/R2 + 1/R3…

This means that two 8- resistors in parallel could be replaced by one 4- resistor.

Parallel CircuitsVoltage drop

In a parallel circuit, the voltage drop is the same across each branch, equal to the battery.

Parallel CircuitsCurrent

In a parallel circuit, the total current in the circuit is equal to the sum of the currents in each branch.

“Rules” for Parallel Circuits

Step 1: Total Resistance = inverse-sum of inverses

1/R = 1/R1 + 1/R2 + 1/R3 + …

Step 2: Voltage drops on each branch = battery

Step 3: Current in each branch uses Ohm’s Law

I1 = V/R1

Step 4: Total current = sum of currents in each branch

I = I1 + I2 + I3 + …

Check: Calculate Total Current using Ohm’s Law

I = V/R

Parallel Circuits

Suppose you have three 3- resistors in parallel connected to a 9-V battery.

What is the total resistance of the circuit?

1

What is the current in the circuit?

9 A

What is the voltage drop across each resistor?

9 V

What is the current in each resistor?

3 A

Parallel CircuitsThree 15- resistors are connected in parallel and placed across a

30-V potential difference. What is the equivalent resistance of the parallel circuit? What is the current through the entire circuit? What is the current through each branch of the parallel circuit?

1/R = 1/R1 + 1/R2 + 1/R3…1/R = 1/15 + 1/15 + 1/151/R = 3/15R = 15/3 = 5

I = V/RI = 30V/5I = 6 A

I1= V/R1

I1 = 30V/15I 1= 2 A

Parallel CircuitsA 120- resistor, a 60- resistor and a 40- resistor are

connected in parallel and placed across a 120-V potential difference. What is the equivalent resistance of the parallel circuit? What is the current through the entire circuit? What is the current through each branch of the parallel circuit?

1/R = 1/R1 + 1/R2 + 1/R3…1/R = 1/120 + 1/60 + 1/401/R = 1/120 + 2/120 + 3/1201/R = 6/120R = 120/6 = 20

I = V/RI = 120V/20I = 6 A

I1= V/R1

I1 = 120V/120I1= 1A

I2= V/R2

I2 = 120V/60I2= 2 A

I3= V/R3

I3 = 120V/40I3= 3 A

Series and Parallel CircuitsAn Analogy

Activity 28What is the effect of adding a bulb in parallel?

Consensus:

Adding a bulb in parallel to a series circuit causes a split of the current to the two parallel branches and an increase in the total current.New Terms:

Combination Circuit – a circuit with both series and parallel parts.

Combination CircuitsCombination circuits – those circuits that contain

both parallel and series portions.

Combination CircuitsThere are steps to take to find the total

resistance:

1) Find the total resistance for any parallel parts.

2) Add that to any resistance in series.

Combination CircuitsSometimes there are more steps to be taken:

1) Start with the smallest section of parallel.

2) Add any in series.

3) Then add as if two resistances in parallel.

4) If there are still some in series, then add those.

Combination CircuitsYou can also do the same thing if each

branch of the parallel portion has some in series:

Worksheet 35-2

circuits chapter 35. lab 21 what will happen to bulbs 1 and 2 when you disconnect the wires at...

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